Schema of Southwest Oncology Group (SWOG) Study S0500. This clinical study is evaluating the use of CTC levels in managing the treatment of metastatic breast cancer patients. Baseline CTC levels are determined using the CellSearch System™. Patients with CTC levels <5 CTCs/7.5 ml receive no further therapy, but are followed for progression-free survival (PFS) and overall survival (OS) (Arm A). For the remaining patients (≥ 5 CTCs/7.5 ml blood), CTCs are measured at specified time points during the course of chemotherapy. Patients with <5 CTCs/7.5 ml blood at 22 weeks continue with their current chemotherapy (Arm B). Patients with ≥5 CTCs/7.5 ml blood at this time point are randomized to current therapy (Arm C1) or a different therapy (Arm C2). Patients are followed for PFS and OS. (Reprinted by permission from the American Association for Cancer Research: Hayes DF, Smerage J: Is there a role for circulating tumor cells in the management of breast cancer? Clin Cancer Res 2008, 14 (12):3646–3650 DOI:http://dx.doi.org/10.1158/1078-0432.CCR-07-4481. See reference).